Structured optical materials including waveguides and gratings form important building blocks for photonic devices for telecommunications, sensing and optical data storage.
Ultra-short pulses provided by femtosecond (fs) lasers create highly localised refractive index changes deep within transparent media, forming sub-micron feature sizes due to non-linearity at high intensities, without requiring photosensitisation by doping or hydrogen loading.
The PSI provides a range of femtosecond lasers with variable wavelength, pulselength and repetition rate that can be used to write structures within transparent optical materials such as glasses and polymers.
Polymers such as polymethyl methacrylate (PMMA) are of interest because it is inexpensive, rugged, and thus ideal for disposable devices for clinical, biological and chemical applications. Low processing temperatures permit organic dopants (unsuited to high TG dielectrics such as glass) that impart optical activity and lasing properties to PMMA and other polymers. Thus amplifiers, fibre-lasers and electro-optic modulators as well as LEDs can be formed on a common polymer substrate, to be combined with fs written 3D waveguide and stacked optical structures such as photonic crystals. Since fs irradiation can form micro-channels and micro-voids, integrated photonic circuits for use as lab-on-the chip devices can be created without etching, lithography or deposition, facilitating polymer based sensing and micro-fluidic devices. PMMA has high elasticity and thermal expansion coefficients when compared with glass materials, and forms the base material for polymer optical fibres (POF). The ability to write periodic refractive index structures (Bragg gratings or long period gratings) into the core of single mode optical fibre can create filters and reflectors that are more sensitive to strain or temperature than equivalent structures written into glass optical fibres.
1D and 2D optical structures for which optimal writing conditions are identified can be combined to form simple demonstrator optical circuits in bulk material, for example Y-splitter, waveguides and 2D gratings, 3D structures and arrays such as photonic crystals. PMMA based injection moulding methods for manufacturing microfluidic devices facilitate lab-on-a-chip, miniaturised analytical sensors with surface moulded optical devices such as lenses and gratings. Incorporating 3D internal optical circuits enables compact, robust, miniature versions of optical instruments inside a PMMA substrate such as miniature spectrometers.
Polymer optical fibre sensors and devices:
Writing periodic refractive index structures into the core of single mode optical fibre creates filters and reflectors that are sensitive to strain or temperature changes (in Fibre Bragg grating-FBG), or enables core modes to interact with cladding measurands (Long Period Grating-LPG). PMMA has high elasticity and thermal expansion coefficients, and is the base material for polymer optical fibres (POF). It has advantages for strain sensing over glass fibre due to its increased sensitivity by 14%,biocompatibility, and good optical transmission in the visible region coincident with high brightness, low cost visible LEDs and laser diodes now commercially available.
Collaboration between The University of Manchester, Liverpool University, Lairdside Laser Engineering Centre and University of Strathclyde.
Unilever-UMIST partnership in advanced measurement:
PhD Studentship in Polymer Waveguide Sensors: formulation, fabrication and femtosecond post processing.
EP/C002059/1 Rfractive index modification of polymers using femtosecond laser irradiation.
EP/D059771/1 Equipment Grant: Femtosecond Laser irradiation of Polymers.